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Anything that holds you back and makes you slow down – or, even worse, stop – your training can be a huge source of frustration. After all, as an athlete, you are all about pushing yourself past your limits. When you start to feel that burning exhaustion in your muscles, though, it can feel like your body is turning on you. What is that terrible internal roadblock and what can you do?

What’s Really Going On

At any given time, your body is using a mix of two different processes to create the ATP your muscles need. The first method, aerobic respiration, uses oxygen to break down glycogen and fat. The other approach, anaerobic respiration, doesn’t require oxygen at all to produce that valuable ATP.

When exercise intensity gets high enough, though, you start to use up more oxygen then you’re breathing in. For example, short but intense activities like sprinting generally fall into the anaerobic category.

But problems happen when demand exceeds supply.

At this point, anaerobic respiration becomes your body’s primary fuel source since it can’t count of a steady oxygen supply. As a byproduct of ATP production, lactic acid is also created. Usually, lactic acid is produced at a slow enough rate that your body can get rid of it without much fuss but things change when anaerobic respiration takes over. Lactic acid is produced too quickly and it quantities that are too high for your body to efficiently clean up.

This point, when your body switches to anaerobic respiration and lactic acid production speeds up, is called the lactic (or anaerobic) threshold. It’s this acidic build-up that your muscles start to burn and shut down because they can’t work as efficiently in the newly-created high acid environment.

The good news is that you can train for this and improve your lactic threshold, allowing you to exercise at higher intensities for longer periods of time.

But first, you have to know what your personal lactic threshold is.

Testing Your Limits

Traditionally, finding your lactic threshold was a big ordeal that required specialized equipment and trained operators. Usually, you would have to pay a laboratory to conduct the test for you.

To try to come up with a inexpensive and more accessible alternative, some coaches have come up with field test that are designed to estimate an athlete’s lactic threshold. While these tests have been shown to be fairly accurate, they also tend to be extremely difficult. For example, one test involves running as far as you can in 30 minutes and taking the average heart for your last 10 minutes of the run.

Fortunately, researchers at Munich Technical University have recently experimented with a test you can do on your own without completely killing yourself. Start out by strapping on a heart rate monitor and setting yourself up to run. You can also cycle if you prefer. After a 2 or 3 minute warmup, gradually start increasing the intensity of your workout. Each time you step it up, note your perceived exertion on a scale of 1 to 10. When you hit 6 on the exertion scale, note your heart rate. That’s your lactic threshold.

Knowing this number for yourself is key in designing endurance training programs. In a future post, we’ll talk more about how to improve your lactic threshold.